Technical Field
The invention relates to a device for thermoforming a plastic film.
Description of the Related Art
It is known to preform a plastic film for application onto a curved surface of a substrate, in order to reduce the stresses produced in this film when it is subsequently assembled with the substrate. In this manner the film can be applied to the curved surface of the substrate without excessive stretching or folds, tears, crushing, or scratches being produced in the film by the tool used to apply it. To this purpose, the plastic film is preformed using a thermoforming device which may comprise:                a chamber with variable internal pressure, which has an opening in a side of this chamber;        an attachment system for attaching the film around the opening, which is adapted to hold a peripheral edge of the film firmly so that the film closes off and seals the chamber;        a system for varying and controlling the pressure in the chamber;        a measurement system for measuring a sag (“straight height”) of the film which is held by the attachment system, this measurement system being arranged outside the chamber and facing the chamber opening; and        a system for heating the film, adapted to heat the film held by the attachment system, and comprising a hot air blowing unit arranged to produce a stream of hot air flowing externally to the chamber and in contact with the film.        
The film which is held by the attachment system is deformed, where it crosses the opening of the chamber, in a deformation which varies with the pressure that is created in the chamber. In this manner, the plastic film can be deformed while subjected to heat, with a deformation amplitude that is controlled while measuring the sag of the film in the opening of the chamber.
It is also known to place the substrate within the inside volume of the chamber, supporting it in a manner which is appropriate for controlling the approach of the substrate to the plastic film while the film is still being held in the opening of the chamber by the attachment system. Thus the thermoforming device also applies the film onto the substrate, which is particularly advantageous when compared to the number of manipulations of the film required to assemble the film and substrate together.
To provide sufficiently precise measurements of the film sag, the sag measurement system must be positioned substantially in alignment with the center of the chamber opening. For this reason, and when the inside volume of the chamber is already occupied by the substrate, this sag measurement system is placed outside the chamber.
In existing thermoforming devices of the type considered, the system for heating the film is also located outside the chamber, next to the film sag measurement system. This heating system commonly uses infrared radiation or blown hot air. The space that is then required, in the area outside the chamber in front of its opening, interferes with the heating of the film. The temperature of the heated film then varies between different points of said film. The deformation of the film produced by the thermoforming is then distributed in a manner which no longer corresponds to the desired shape. In particular, the distribution of this deformation is no longer rotationally invariant around an axis perpendicular to the side of the chamber which contains the opening, and which passes through the center of this opening. Certain of the defects mentioned above then reappear when a film preformed in this manner is applied onto the substrate, particularly when the substrate face is substantially spherical in shape.
In particular, such thermoforming devices are used to preform a plastic film that is intended to be applied onto an optical lens. The quality requirements of the final optical component are then particularly high, especially when it is a component such as a spectacle lens or a lens blank. For this type of application, the end product is rejected when stretching, folds, tears, crushing, or scratches in the film are visible to the naked eye. Thus for optical and ophthalmic applications in particular, the thermoforming devices currently in use are unsatisfactory.